The present invention relates generally to oscillator circuits, and more particularly to a low noise oscillator circuit which employs a self-biasing JFET amplifier.
A feedback oscillator may broadly be defined as an amplifier whose output is fed, in phase, to its input through a frequency selective circuit (usually a high-Q tank circuit). Feedback oscillator designs are known which utilize junction field-effect transistors (JFET's) as the amplification element. In general, oscillation amplitude stability is provided in these circuits by constructing them so that the degree to which the JFET loads the tank circuit increases with increasing amplitude of oscillation. Thus, as the level of oscillation increases, the loading of the tank circuit by the JFET also increases. This, in turn, results in a reduction in the loop gain of the oscillator. Eventually, a point is reached at which the loop gain is reduced to unity, thus establishing oscillatory equilibrium.
Although this technique is quite acceptable for many uses, it has the disadvantage that the output signal is rather noisy. In many applications, however, it is highly desirable that amount of noise in the output signal be reduced to as great a degree as possible. In multichannel communication systems, for example, it is desirable that the noise sideband profile of the carrier oscillator be reduced to as low a level as possible so as to minimize both on channel, and adjacent-channel noise.
Other JFET oscillator circuits are known which do not rely upon tank circuit loading in order to establish oscillatory equilibrium. These oscillators, known as self-limiting JFET oscillators, utilize the gate-to-source junction of the JFET in order to provide peak rectification of the gate signal. A capacitor and resistor are connected in the gate circuit in order to filter this rectified signal and thereby develop a gate-biasing signal. Oscillators of this type are described on pages 241-243 and 131-137 of the book entitled "Communication Circuits: Analysis and Design", published in 1971 by the Addison-Wesley Publishing Company. The capacitor utilized in the gate biasing circuit of these oscillators is selected to be an AC short circuit at the oscillation frequency, whereas the resistor is selected to be in the range of one to ten megohms in order to prevent any significant loading of the tuned circuit.
This oscillator circuit, while not producing the amounts of noise present in oscillator circuits which rely upon the loading of the tank circuit to establish equilibrium, nonetheless still includes some amount of undesirable noise at the output thereof.